1. Field of the Invention
The present invention relates to thermosetting resin to compositions with excellent electrical properties; electrical laminates made therefrom; and methods of producing these.
2. Background of the Invention
All publications and patent applications herein are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.
Electrical laminates such as circuit boards are produced by laminating sheets of electrical conducting material onto a base substrate of insulation material. The performance of the finished circuit board is effected by the electrical characteristics of the base substrate material.
Commercially available thermoset resin systems with acceptable electrical performance at high frequencies ( greater than 350 MHZ) are restricted in their applications due to high cost. The lower cost alternatives that are available do not perform satisfactorily at high frequencies due to unacceptable electrical properties, such as high dielectric constant (Dk), high dissipation factor (Df), high variability of Dk and Df with frequency, and consistency of Dk and Df from lot to lot of production material.
Thermoplastic polymers such as polytetrafluoroethylene (PTFE) which have exceptional electrical performance at high frequencies are commercially available. The primary drawbacks associated with these materials are very high raw material costs and special processing considerations that add substantial cost to the final product. Because of the physical properties, very high laminating temperatures and pressures are also required to fabricate an electrical laminate from PTFE. Furthermore, due to the inability to xe2x80x9cwetxe2x80x9d PTFE, costly and hazardous chemicals are required to modify its surface during fabrication of the circuit.
A thermoset material would have much greater mechanical properties over a much broader temperature range. Additionally, since thermoset materials have better mechanical properties, this would allow the circuit board fabricator to use conventional cost effective processes.
A need exists for thermosetting resin compositions, and electrical laminates made therefrom, of low to moderate cost with acceptable electrical properties at frequencies up to at least 20 GHz. Such compositions would have great utility as circuit board substrates and the laminates made from these thermosetting resins can be utilized in many applications such as in the rapidly growing wireless communication market, in high speed computers, in high definition televisions, and in various other electrical and related applications.
An object of the present invention is to provide a thermosetting resin composition which can be made flame retardant and which can be utilized in existing cost effective technologies to manufacture electrical laminates therefrom. Additionally, the present invention resin compositions can have utility as, for example, electrical insulators, encapsulants, insulating adhesives and in various other electrical and related applications generally know to those skilled in this art.
Accordingly, the present invention provides a thermosetting resin composition comprising (a) a terminally unsaturated urethane resin selected from the group of 1): 
where R1 is H or CH3, m is 2 or 3, and R3 is an organic residue from a diisocyanate; and 2): 
where R1 is H or CH3, m is 2 or 3, R3 is an organic residue from a diisocyanate, and R4 is an isocyanurate compound of the following structure; 
and (b) an ethylenically unsaturated monomer from the group of 1) styrene and 2) bromostyrene with the following structures: 
where m=1 to 3.
Additionally, to achieve the desired properties (i.e., microwave transparency, fire retardation, and good thermal performance) the ratio of (a) to the sum of (b)1) and (b)2) is less than 0.15, and the ratio of (b)1) to (b)2) is less than 1.2.
The composition resins may also further contain monomers which would, for example, aid in the adhesion of a metal foil to the laminate; increase crosslink density and thermal performance; etc. Catalysts which, for example, induce free-radical cure may also be added. Other components may include moisture scavengers, compounds which increase or reduce the dielectric constant, and/or reduce the dissipation factor, polyethylene fillers, other fillers, organic or inorganic, which modify rheology, surface agents, viscosity and performance modifiers, wetting agents, air release agents, defoaming agents, flame retardant synergists, adhesion promoters, and other additional monomers and conventional additives known in the art.
Other objects of the present invention are to provide electrical laminates, and methods for producing these laminates containing the composition resins of the present invention, which have excellent electrical properties and which may be flame retardant or heat resistant. Accordingly, the present invention further provides electrical laminates from about 0.003 inches to about 0.120 inches thick which may or may not be clad with an electrical conducting material on one or both sides.
The electrical laminates of the present invention are produced by (a) continuous lamination or (b) a batch process:
(a) The compositions of the present invention are uniquely suitable to the continuous lamination methods of Barrel et al. (U.S. Pat. Nos. 4,587,161 and 4,803,022). The combination of materials in the outlined ratios provides lower viscosities then previous compositions, (U.S. Pat. Nos. 4,420,509 and 4,446,173). Lower viscosities provide more rapid impregnation of reinforcements to allow higher line-speeds and consequently higher machine output. In the continuous lamination process, the desired composition is formulated with a free-radical source, (i.e., peroxide, azo-compound, etc.) to initiate cure. Optionally, the material is postcured in a batch convection oven to further lower the dissipation factor of the laminate. Therefore, we have found the combination of continuous lamination with additional batch cure provides unexpected improvements in electrical performance. While the continuous lamination temperatures and times are known in the art, we have found the optimum secondary batch oven cure profile to be 350xc2x0 F. for 1 hour dwell, (or range from 150xc2x0 F. for 3 hours to 450xc2x0 F. for 30 minutes).
(b) The method to manufacture the laminate with a batch process involves
1) A layer of carrier film, such as polyethylene terephthalate of 1.42 mills thick, was placed on a xc2xcth inch thick glass plate of 1.25 by 1.25 ft. The dimensions of the film were large enough to protrude around the edges of the glass plate.
2) A one foot square piece of 1 oz. per sq. foot weight of copper foil was placed treatment side up on the carrier film.
3) A film of a resin composition, prepared according to the present invention, was metered onto the copper foil by using a wire-wound rod designed to provide a coating of the target thickness of the laminate. Wire-wound rods are well known throughout the coatings industry.
4) A layer of glass cloth, woven or nonwoven, was laid onto the resin film and allowed to saturate for approximately 2 minutes. If a plurality of layers were used, the layers were placed onto the resin film approximately 2 minutes apart to allow the resin mixture to saturate the glass.
5) An additional layer of copper foil, of the same size and weight as 2), was placed treatment side down on the laminate so as to align the edges with the first sheet of copper foil.
6) Another layer of carrier film, of the same size and dimensions as 1), was placed on top of the copper foil.
7) Two xc2xd inch wide by 12 inches long shims of the target laminate thickness were placed on opposing sides of the laminate, on top of the carrier film, but still on the glass plate.
8) A rod of xc2xd inch thick steel was placed on the shims at one edge of the laminate and gently pulled to the opposing edge while being forced by hand to ride on the shims. As the rod moved across the laminate, excess resin composition was allowed to drain out of the laminate.
9) Another plate of glass of equivalent dimensions as 1), was placed on top of the laminate.
10) The laminate was placed in a forced air convection oven at 150-250xc2x0 F. for 15 minutes to 1 hour.
11) The laminate, still between the glass plates, was removed from the oven and allowed to cool to room temperature. At this time, the two glass plates and two layers of carrier film were removed.
12) Post cure from 350xc2x0 F. for 1 hour in a forced air convention oven.
The thermosetting resin composition may also further contain the above mentioned added components and may be cured by electron beam processing, radiation, heat with or without pressure, ultra violet light processing, and other conventional curing methods in conjunction with the appropriate initiators. The electrical laminates may further comprise an electrical conductive cladding on at least one side.
One skilled in the art can easily make any necessary adjustments in accordance with the necessities of the particular situation.
Further objects and advantages of the present invention will be clear from the description that follows.